Getting a visa of any kind to work in NZ is getting more and more difficult at the moment. Partly for internal political reasons, they are cutting down the number of migrants accepted, so the criteria are becoming more demanding. For a would-be migrant, it is all about showing yourself and your qualifications and experience in a way that fits into the INZ regulations.
https://www.immigration.govt.nz/new-zealand-visas There aren't any easy ways. NZ only want to admit foreigners who have niche skills that are in short supply among their own citizens - they have plenty of people who can do unskilled jobs that only require minimal training.
Your brother-in-law, in order to get a visa these days, would probably need to be offered a job for which he is fitted by his qualifications and experience. It has become very rare to find a situation where INZ are willing to admit that there aren't any NZers who could do a lower-skilled job, and so grant a visa to a foreigner to do it.
https://www.immigration.govt.nz/new-...ills-work-visa
If he is offered a NZ work visa, his partner can apply for a partner-sponsored temporary work visa
https://www.immigration.govt.nz/new-...k-visa-holders, and visitor's visas to match the parents' for length of time if they are under 5 for the children, or student visas if they are 5 and over.